We propose studies to examine those interactions between certain animal viruses and their host cells which result in selective interference of protein synthesis directed by either cellular or viral messenger RNAs. As an example of virus-induced inhibition of cellular protein synthesis, we are utilizing poliovirus infection of cultured HeLa cells, which results in a rapid disaggregation of host cell polysomes prior to the formation of poliovirus-specific polysomes and uninhibited translation of viral mRNA. Two examples of host cell restriction of virus replication are being studied: abortive infection of monkey cells with human adenovirus type 2, and restrictive infection of human lymphoblastoid cells with vesicular stomatitis virus. In all cases, mRNA synthesis has been shown to occur, but translation is blocked at the level of initiation. We are examining both cellular and viral protein synthesis in vitro to determine the differences in requirements for host cell and viral protein synthesis and to analyze the factors causing restriction. We are analyzing the fate of mRNA where translation is inhibited in vivo, and we are comparing the functionality of mRNA synthesized under restrictive and permissive conditions in vitro. We hope to understand the mechanisms by which some cells can restrict virus growth, and by which selectivity for different mRNAs can be imposed on the process of protein synthesis.